Recently, efforts have been made to prepare heterophasic copolymers, such as impact copolymers (ICP), using metallocene or single site catalysis technology to capitalize on the benefits such catalysts provide. Homopolymers (such as isotactic polypropylene, iPP) prepared with such single-site catalysts often have a narrow molecular weight distribution (MWD), low extractables, and a variety of other favorable properties associated therewith, and copolymers (such as ethylene propylene rubber, EPR) prepared with such single-site catalysts often also have narrow composition distributions.
Unfortunately, metallocenes, immobilized on a conventional support (such as silica) coated with an activator (such as methylalumoxane, MAO), are typically not able to provide dispersed phases of EPR components (such as in the case of iPP/EPR impact copolymers), with sufficiently high molecular weight and/or rubber loadings under commercially relevant process conditions. Compared to their Ziegler-Natta system catalyzed counterparts, isotactic polypropylene (iPP) matricies in impact copolymers prepared using metallocenes and or other single site catalysts typically have a low porosity, and are unable to hold a sufficiently high rubber (such as EPR) content within the iPP matrix required for toughness and impact resistance. Also, the EPR often has an MWD that is too narrow to obtain a good balance between the desired melt flow rate for material processing and stiffness. This is thought to be due to formation of rubber in a separate phase outside the matrix, which is believed to result in reactor fouling.
Pore structures in conventional iPP, whether from Ziegler-Natta or metallocene systems, are thought to be generated from the fast crystallization of low molecular weight portions of the polymer that causes volumetric shrinkage during crystallization. See Nello Pasquini (Ed.), Polypropylene Handbook, 2nd Edition, Hanser Publishers, Munich, pp. 81-88 (2005). Likewise polymer particle morphology is thought to be directly related to catalyst support properties. See Cecchin, G. et al., Marcromol. Chem. Phys., vol. 202, p. 187, (2001).
Accordingly, it has been elusive to balance the toughness and stiffness polymer properties in a single-catalyst sequential polymerization process to make impact copolymer. On the one hand, the formation of high porosity and high fill rubber loading needed for toughness has often required the presence of a high concentration of hydrogen to form low molecular weight polymers needed for the fast-crystallization shrinkage, and on the other hand, polymerization under these conditions for maximizing porosity detracts from the stiffness of the resulting ICP.
Approaches have been attempted to solve this problem, see, for example, U.S. Pat. No. 5,990,242; WO 2004/092225; EP 1 380 598; EP 1 541; EP 1 205 493; JP 2003073414; and JP 2012214709.
Other references of interest include: US 2011/0034649; US 2011/0081817; Madri Smit et al., Journal of Polymer Science: Part A:Polymer Chemistry, Vol. 43, pp. 2734-2748 (2005); and “Microspherical Silica Supports with High Pore Volume for Metallocene Catalysts,” Ron Shinamoto and Thomas J. Pullukat, presented at “Metallocenes Europe '97 Dusseldorf, Germany, Apr. 8-9, 1997.
Accordingly, there is need for new catalysts and/or processes that produce polypropylene materials including impact copolymers that meet the needs for use in commercial applications, such as one or more of: a good balance of stiffness and toughness, and/or other properties needed for high impact strength; homopolymers and copolymers with narrow MWD, low extractables, bimodal MWD, bimodal particle size distribution (PSD), narrow composition distribution, and/or other benefits of single site catalyzed homopolymers and copolymers; high porosity propylene polymers; heterophasic copolymers with a high dispersed phase content of a second polymer component in a first polymer component; preparation of bimodal MWD or bimodal PSD heterophasic copolymers in a single-catalyst, sequential polymerization process; economic production using commercial-scale processes and conditions; and combinations thereof.
It has been observed that the weight average molecular weight (Mw) of rubber (such as EPR) in impact copolymers is an important factor for determining the toughness and the processability of an ICP. Further, it has been observed that a higher Mw usually increases the toughness of the ICP but decreases its processability.
Accordingly, there is particular need for new catalyst systems, particularly metallocene catalyst systems, having the capability of producing heterophasic copolymers such as impact copolymers, which can have balanced loading and controlled molecular weight of the dispersed phase, more desirably, having the capability of generating both high and low Mw portions in the dispersed phase to simultaneously influence the toughness and the processability of the impact copolymers.